Well region formation is an indispensable process in semiconductor manufacturing.
A well region formation method I the prior art will be described with reference to the figures. The method comprises the following steps.
As shown in FIG. 1, a semiconductor substrate 100 is provided, and the semiconductor substrate 100 comprises: a first isolation structure 111, a second isolation structure 112, a third isolation structure 113, a fourth isolation structure 114 and a fifth isolation structure 115. The isolation structures are for isolating active regions.
As shown in FIG. 2, a photoresist 120 is formed on the semiconductor substrate 100 outside an active region that is between the second isolation structure 112 and the third isolation structure 113.
As shown in FIG. 3, implantation of dopant ions 140 is performed.
As shown in FIG. 4, a first well region 151 is formed in the active region between the second isolation region 112 and the third isolation structure 113, and the depth of the first well region 151 is generally greater than the depth of the isolation structures.
As semiconductor devices are scaled further, the isolation structures continue to shrink. Therefore, in the formation of the first well region 151. because of the small size of the isolation structures, lateral scattering of some of the dopant ions occurs, i.e., some of the dopant ions pass through the second isolation structure 112 and the third isolation structure 113, forming a first inadvertently doped area 161 along the upper and middle parts of a side of the second isolation structure 112 that faces the first isolation structure 111, as well as a second inadvertently doped area 162 along the upper and middle parts of a side of the third isolation structure 113 that faces the fourth isolation structure 114; and the first inadvertently doped area 161 and the second inadvertently doped area 162 both have a doping type the same as the first well region 151.
As shown in FIG. 5, the photoresist 120 is removed, and a semiconductor device comprising the first well region 151, the first inadvertently doped area 161 and the second inadvertently doped area 162 has been formed.
As shown in FIG. 6, by the same method, second well regions 152 are formed in an active region between the first isolation structure 111 and the second isolation structure 112, an active region between the third isolation structure 113 and the fourth isolation structure 114, and an active region between the fourth isolation structure 114 and the fifth isolation structure 115.
Similarly, in the formation of the second well regions 152, a third inadvertently doped area 163 is formed along the upper and middle parts of a side of the second isolation structure 112 facing the third isolation structure 113; a fourth inadvertently doped area 164 is formed along the upper and middle parts of a side of the third isolation structure 113 that faces the second isolation structure 112; a fifth inadvertently doped area 165 is formed along the upper and middle parts of a side of the fourth isolation structure 114 that faces the third isolation structure 113; and a sixth inadvertently doped area 166 is formed along the upper and middle parts of a side of the fourth isolation structure 114 that faces the fifth isolation structure 115. The third inadvertently doped area 163, the fourth inadvertently doped area 164, the fifth inadvertently doped area 165 and the sixth inadvertently doped area 166 all have a doping type the same as the second well region 152.
The doping type of the first well region 151 is different from that of the second well region 152. For example, the first well region 151 is N-type doped and the second well region 152 is P-type doped. Then, the first inadvertently doped area 161 and the second inadvertently doped area 162 are N-type doped, and the third inadvertently doped area 163, the fourth inadvertently doped area 164. the fifth inadvertently doped area 165, and the sixth inadvertently doped area 166 are P-type doped.
Because the fifth inadvertently doped area 165, the sixth inadvertently doped area 166 and the second well region 152 have the same doping type, the fifth inadvertently doped area 165 generally will not affect threshold voltage of the semiconductor device between the third isolation structure 113 and the fourth isolation structure 114, and the sixth inadvertently doped area 166 generally will not affect threshold voltage of the semiconductor device between the fourth isolation structure 114 and the fifth isolation structure 115.
However, the first inadvertently doped area 161 and the second well region 152 have different doping types, hence, threshold voltage of the semiconductor device between the first isolation structure 111 and the second isolation structure 112 will change. Similarly, because the third inadvertently doped area 163 and the first well region 151 have different doping types, and the fourth inadvertently doped area 164 and the first well region 151 have different doping types, threshold voltage of the semiconductor device between the second isolation structure 112 and the third isolation structure 113 will change; because the second inadvertently doped area 162 and the second well region 152 have different doping types, threshold voltage of the semiconductor device between the third isolation structure 113 and the fourth isolation structure 114 will change. Threshold voltage change will lead to affected performance of the semiconductor device.
Therefore, it is desired to reduce the threshold voltage change of the semiconductor device caused by lateral scattering of dopant ions in the formation of the well region.